Despite the desperate need for an HIV vaccine, current approaches towards a vaccine has had limited success. We have shown that a major factor in the resistance of many Envs derived from primary viruses to neutralization by standard antibodies is masking of sensitive neutralization targets by the V1/V2 domain. This suggests that the development of an effective vaccine would be facilitated by a better understanding of this resistance mechanism, and by the identification of additional immune targets that are not susceptible to such blocking effects. We have identified a particular region in the V2 domain of primary HIV-1 Envs that is an extremely sensitive neutralization target to several mAbs. C108g, directed against a conformational and glycan-dependent V2 epitope, neutralizes viruses containing its epitope with greater potency than that of almost any other known antibody. 2909, directed against a quaternary epitope specifically expressed on intact virions but not soluble Env proteins, possesses even more potent neutralizing activity for SF162 and a JR-FL variant. 2909 binding to virions requires the presence of both the V2 and V3 domains in Env. We have recently shown that the V3 determinants of the 2909 epitope are very broad, and that the type-specificity of 2909 is due to the requirement for residues in V2 that map to precisely the same region as the C108g determinants. Although these mAbs are type-specific, both the C108g and 2909 epitopes differ from the consensus clade B V2 sequence only at single residues. The unrivalled potency of these mAbs argues that they bind to a particularly sensitive neutralization domain, and suggests that conserved forms of these epitopes could be important vaccine targets. This proposal will further map V1/V2 determinants responsible for neutralization masking, examine the role of the V1/V2 domain in autologous neutralization, more completely define epitopes in V2 that mediate potent neutralization, isolate and characterize additional V2- specific mAbs with broader neutralizing activities and determine the structures of complexes formed between such Fabs and appropriate V1/V2 antigens. Finally, this data will be used to design optimized miniproteins that present the critical V2 epitopes in highly immunogenic forms and to test the ability of such immunogens to induce broadly neutralizing antibodies in several animal models.